sasl_sspi: Allow DIGEST-MD5 to use current windows credentials

Fixed the ability to use the current log-in credentials with DIGEST-MD5.
I had previously disabled this functionality in commit 607883f13c as I
couldn't get this to work under Windows 8, however, from testing HTTP
Digest authentication through Windows SSPI and then further testing of
this code I have found it works in Windows 7.

Some further investigation is required to see what the differences are
between Windows 7 and 8, but for now enable this functionality as the
code will return an error when AcquireCredentialsHandle() fails.

docs/curl.1

@@ -1679,9 +1679,9 @@ Principal Name) formats. For example, EXAMPLE\\user and user@example.com
 respectively.
 
 If you use a Windows SSPI-enabled curl binary and perform Kerberos V5,
-Negotiate or NTLM authentication then you can tell curl to select the user
-name and password from your environment by specifying a single colon with this
-option: "-u :".
+Negotiate, NTLM or DIGEST-MD5 authentication then you can tell curl to select
+the user name and password from your environment by specifying a single colon
+with this option: "-u :".
 
 If this option is used several times, the last one will be used.
 .IP "-U, --proxy-user <user:password>"

lib/curl_sasl_sspi.c

@@ -128,6 +128,7 @@ CURLcode Curl_sasl_create_digest_md5_message(struct SessionHandle *data,
   CtxtHandle ctx;
   PSecPkgInfo SecurityPackage;
   SEC_WINNT_AUTH_IDENTITY identity;
+  SEC_WINNT_AUTH_IDENTITY *p_identity;
   SecBuffer chlg_buf;
   SecBuffer resp_buf;
   SecBufferDesc chlg_desc;
@@ -147,13 +148,6 @@ CURLcode Curl_sasl_create_digest_md5_message(struct SessionHandle *data,
   if(!chlg)
     return CURLE_BAD_CONTENT_ENCODING;
 
-  /* Ensure we have some login credentials as DigestSSP cannot use the current
-     Windows user like NTLMSSP can */
-  if(!userp || !*userp) {
-    Curl_safefree(chlg);
-    return CURLE_LOGIN_DENIED;
-  }
-
   /* Query the security package for DigestSSP */
   status = s_pSecFn->QuerySecurityPackageInfo((TCHAR *) TEXT("WDigest"),
                                               &SecurityPackage);
@@ -185,25 +179,33 @@ CURLcode Curl_sasl_create_digest_md5_message(struct SessionHandle *data,
     return CURLE_OUT_OF_MEMORY;
   }
 
-  /* Populate our identity structure */
-  result = Curl_create_sspi_identity(userp, passwdp, &identity);
-  if(result) {
-    Curl_safefree(spn);
-    Curl_safefree(resp);
-    Curl_safefree(chlg);
+  if(userp && *userp) {
+    /* Populate our identity structure */
+    result = Curl_create_sspi_identity(userp, passwdp, &identity);
+    if(result) {
+      Curl_safefree(spn);
+      Curl_safefree(resp);
+      Curl_safefree(chlg);
 
-    return result;
+      return result;
+    }
+
+    /* Allow proper cleanup of the identity structure */
+    p_identity = &identity;
   }
+  else
+    /* Use the current Windows user */
+    p_identity = NULL;
 
   /* Acquire our credentials handle */
   status = s_pSecFn->AcquireCredentialsHandle(NULL,
                                               (TCHAR *) TEXT("WDigest"),
                                               SECPKG_CRED_OUTBOUND, NULL,
-                                              &identity, NULL, NULL,
+                                              p_identity, NULL, NULL,
                                               &handle, &expiry);
 
   if(status != SEC_E_OK) {
-    Curl_sspi_free_identity(&identity);
+    Curl_sspi_free_identity(p_identity);
     Curl_safefree(spn);
     Curl_safefree(resp);
     Curl_safefree(chlg);
@@ -237,7 +239,7 @@ CURLcode Curl_sasl_create_digest_md5_message(struct SessionHandle *data,
     s_pSecFn->CompleteAuthToken(&handle, &resp_desc);
   else if(status != SEC_E_OK && status != SEC_I_CONTINUE_NEEDED) {
     s_pSecFn->FreeCredentialsHandle(&handle);
-    Curl_sspi_free_identity(&identity);
+    Curl_sspi_free_identity(p_identity);
     Curl_safefree(spn);
     Curl_safefree(resp);
     Curl_safefree(chlg);
@@ -254,7 +256,7 @@ CURLcode Curl_sasl_create_digest_md5_message(struct SessionHandle *data,
   s_pSecFn->FreeCredentialsHandle(&handle);
 
   /* Free the identity structure */
-  Curl_sspi_free_identity(&identity);
+  Curl_sspi_free_identity(p_identity);
 
   /* Free the SPN */
   Curl_safefree(spn);